1. Field of the Invention
The present invention is related to an organic light emitting device (LED) and a method of producing an organic LED which possesses a stable light emission with a high efficiency. The present invention can be used for LEDs in display, a back light for a liquid crystal display, a flat light source, a flat panel display, etc.
2. Description of the Background
Diamond is known to have excellent resistance to high temperatures and has a large band gap (5.5 eV). Furthermore, diamond has excellent electrical properties such that the breakdown voltage is high, the saturation velocities of carriers (electrons and holes) are also high, and the dielectric constant, and hence the dielectric loss, is small. It is also well known that diamond has both the highest thermal conductivity among all materials at room temperature and a small specific heat.
Regarding chemical vapor deposition (CVD) of diamond films, the following techniques are known: microwave plasma CVD (for example, see Japanese patents (kokoku) Nos. Sho 59-27754 and Sho 61-3320), radio-frequency plasma CVD, hot filament CVD, direct-current plasma CVD, plasma-jet CVD, combustion CVD, and thermal CVD. By those techniques, it is possible to form continuous diamond films over large areas at low cost on substrates which consist of non-diamond materials.
To deposit a diamond film by CVD, hydrocarbon gas, such as methane, diluted with hydrogen is used as the source gas, and an electrically insulating diamond film can be formed. It is also well known that a p-type semiconducting diamond film can be formed by adding a gas comprising boron (B) atoms, such as diborane(B.sub.2 H.sub.6), in the source gas.
Recently, a field-emission (FE)-type light emitting element, which consists of an electrode coated with a fluorescent material that is facing to a diamond film in vacuum, was proposed. The element uses a light emission from the fluorescent material excited by electrons which have been injected into the vacuum from the diamond film and accelerated under a high voltage between the diamond film and the fluorescent electrode.
However, in the FE-type light emitting element, vacuum is necessary, and further there are problems that the structure of the device and the process to manufacture the device are complicated. Moreover, a high voltage, e.g., 5 to 10 kV, must be applied between the diamond film and the fluorescent material to accelerate electrons in the operation of the element. Therefore, it is necessary that the system containing the FE-type light emitting element must handle a high voltage, and thus the cost of the system is high.
To solve those problems, an organic LED which possesses a light emission at low voltage was proposed. This is referred to as Prior Art 1. FIG. 4 shows a cross-sectional view of such an organic LED. As shown in FIG. 4, an electrode for hole injection 12 is formed on a glass substrate 11. On the electrode 12, a hole drift layer 13 and a hole injection layer 18 are formed successively. On the hole injection layer 18, an organic light emitting layer 14 and an electron drift layer 15 are formed successively. Further, an electrode for electron injection 16 is formed on the electron drift layer 15. Note that the hole injection layer 18 and the electron drift layer 15 can be omitted depending on the specification of the LED.
As an organic compound to compose the organic light emitting layer 14, distyreallylene group, oxadiazole group, pyrazoloquinoline group, benzooxazole Zn compound group, and aluminum chelate compound can be used. Alternatively, polymer groups such as polyalkylthiophene, polyparaphenylenevinylene, polynaphthalene-vinylene, polyalkylfluorene, polyphenylene, and polymethyl-phenvlsilane can also be used.
Either a transparent electrode film, such as ITO (Indium-Tin-Oxide) or a metal film, such as Au and Ni, which has a large work function, is used as an electrode for hole injection 12. As the electrode for electron injection 16, a material film which has a small work function, such as Mg, Li, and Ca group, or their alloys with Ag or Al, is used. An organic compounds using the amine group is used for the hole drift layer 13, and an organic compound film including the amine group or the phutalocyanine group is used for the hole injection layer 18. As the electron drift layer 15, a film including an aluminum chelate is used.
In the organic LED which was configured as described above, electrons are injected from the electrode 16, and at the same time, holes are injected from the electrode 12. These electrons and holes recombine in the organic light emitting layer 14 and cause a light emission 17 from the side of the glass substrate II. The organic LED according to Prior Art 1 is characterized by the fact that the light emission is achieved at low voltage, and an arbitrary color of light can be obtained by selecting the organic compound to compose the organic light emitting layer 14.
However, there is a problem in the organic LED according to Prior Art 1 that the brightness of light emission is lower than that of the LED utilizing semiconductor, plasma, or field emission. This is attributed to the fact that the hole drift layer 13 and the electron drift layer 15 are composed of organic compounds. In particular, when an organic compound is used as a material for the hole drift layer 13, the drift mobility and carrier life time are low and hence the brightness of light emission also is low. When the amount of current is increased to increase the brightness of light emission, the temperature of the organic light emitting layer 1.4 reaches above over 100.degree. C. This causes a problem that the organic compound layers composing the organic LED are thermally deteriorated.
An organic LED which possesses a high brightness at low current, was proposed in Japanese patent (kaikoku) Hei 6-111938. This will be referred to as Prior Art 2. The structure of the LED is similar to that of the organic light emitting element shown in FIG. 4, and will be explained using the figure. As shown in FIG. 4, an electrode for hole injection 12 is formed on a glass substrate 11 and then a hole drift layer 13 is formed on the electrode 12. An organic light emitting layer 14 and an electron drift layer 15 are successively formed on the hole drift layer 13. Further, an electrode for electron injection 16 is formed on it. A hole drift layer 13 consists of a boron-doped diamond layer in Prior Art 2.
In the organic LED configured as described above, holes injected from the electrode for hole injection 12 and electrons injected from the electrode for electron injection 16 recombine in the organic light emitting layer 14 and the light 17 is emitted from the side of the glass substrate 1. In this case, thermal deterioration of the hole drift layer 13 can be avoided, because the hole drift layer 13 consists of a diamond layer which is resistant to high temperature. In the diamond film, carrier (electron and hole) mobility is high. For example, the hole mobility in diamond ranges from 500 to 1870 cm.sup.2 /V-s. Thus, many holes can be transported to the organic light-emitting layer 14 through the hole drift layer 13 composed of a diamond film, and the light emission efficiency can be improved. Moreover, since the bandgap of diamond is larger than the energy level of the exciton (electron-hole pair) created by the recombination of injected electrons and holes, the exciton can not be annihilated by drifting to the diamond film. This mechanism also increases the light emission efficiency.
However, even in the case that the hole drift layer is a diamond film as shown in Prior Art 2, the light emission efficiency can not be improved sufficiently without choosing proper growth conditions for diamond film. For instance, in Prior Art 2, the diamond film for the hole drift layer was deposited by hot filament CVD. Because of this, the deposited diamond films contain the filament materials (W or Ta) as impurities which cause crystal defects in diamond and hence reduce the light emission efficiency.
In Prior Art 2, an ITO film as an electrode for hole injection 12 is deposited on a glass substrate 1 and the diamond film as a hole drift layer is formed on the ITO film. However, for diamond CVD, the substrate temperature must be higher than 750.degree. C. in the atmosphere of chemically active hydrogen gas. Under these conditions, the ITO film is deteriorated or peeled off from the glass substrate during the diamond CVD. Even if a diamond film is deposited on the ITO film, the diamond film becomes polycrystalline, and since the carrier mobility of polycrystalline diamond film is known to be very low, the light emission efficiency can not be sufficiently improved.
The present invention is proposed to solve those difficult problems. It is an object of the present invention to provide an organic LED and a method of producing the organic LED which can make possible a stable light emission with a high efficiency without thermal deterioration of the hole drift layer.